Along with the tremendous developments in the micromachining technology, the measurement of micro velocities has become an important technology. Among the measurement technologies, the "thermal pulsed micro flow sensor" is widely used in measuring the velocity of a flow in a microchannel.
A conventional thermal pulsed micro flow sensor measures the velocity of a flow according to the "flying time" of a thermal pulse from one point to another. During the measurement, a heater generates a thermal pulse and outputs it to a flowing fluid. While the fluid flows, the thermal pulse is carried by the fluid along a fluid channel. Two thermal sensors are used to sense the flying time of the thermal pulse between them. Since the velocity of the fluid is in inverse proportion to the flying time of the thermal pulse, the velocity may be easily measured from the flying time.
FIG. 1 illustrates the principal of velocity measurement used in a thermal pulsed micro flow sensor. As shown in this figure, a thermal pulse 101 is carried by a fluid flow F. The thermal pulse 101 travels through two thermal sensors and thus two thermal signals 102 and 106 are generated by the sensors separately. The time difference 103 of the two thermal signals 102 and 106 is counted based on a working frequency 104. Velocity of the fluid flow is calculated according to the following equation: EQU F=K*(A*L)/T=K*V/T (1)
In this equation, F represents velocity of the fluid, K represents a calibration factor, A represents the cross sectional area of the microchannel, L represents the distance between the two thermal sensors, T represents time difference between the two thermal signals, i.e., the flying time of the thermal pulse, and V represents volume of the microchannel between the two thermal sensors.
The measurement of micro flows adopting thermal pulses has been disclosed by Harrington et al., in their U.S. Pat. No. 4,782,708 in 1988. In the Harrington invention, disclosed was a micro flow sensor with thermal pulses generated by a resistor driven by an oscillator current source. Two thermal couplers are used to generate respective thermal signals when a thermal pulse generated by the heater passes them, respectively. The time difference of the two thermal signals is measured so that the velocity of the flow may be known.
In 1993 Erskine et al. disclosed a micro flow sensor in their U.S. Pat. No. 5,243,858. In the Erskine invention, only one thermal sensor made of thermister is used. The flying time of the thermal pulse from the heater and the thermal sensor is measured. Velocity of the flow is calculated according to the flying time difference. In order to improve the accuracy in the measurement, two sets of flow sensors are arranged normally to each other. The two-dimensional flow rate of the microchannel is then taken for consideration.
In the above thermal pulsed micro flow sensors, measurements are based on the flying time of the thermal pulses. As a result, the ambient temperature won't affect the accuracy of the measurement. This approach is suited in cases where viscosity or thermal properties of the fluid varies from time to time, or where particles are carried in the fluid, such as in the blood.
In order to maintain the performance of a thermal pulsed micro flow sensor, the thermal pulse 101 shall have a certain level of amplitude and a sufficient width. This is because a pulse attenuates during the flow. In addition to this, variations in thermal conductivity or in flow will bring distortions into the pulse. These and other factors influences the accuracy of measurement of the micro flow.
FIG. 1a illustrates the relation between a thermal signal and the accuracy of measurement in a thermal pulsed micro flow sensor. In this figure, 105 represents width of a pulse 101 after being distorted. Accuracy in sensing the distortion and in picking-up the signals is limited to approximately the average width of the thermal pulses. In general case, the width is about 70-100 .mu.s.
On the other hand, requirements for a measurement instrument include: broader measurable scale, high resolution and short response time. These requirements are in conflict with the said limitation of accuracy. Solutions to such conflict has then become a major task in the thermal pulsed micro flow sensor.
Take a mass flow controller adopting a thermal pulsed micro flow sensor as an example. FIG. 2 illustrates the structure of such a mass flow controller. As shown in this figure, a mass flow controller of this kind generally includes a fluid channel 205, a bypass microchannel 204, a flow sensor 201, a controller 203 and a microvalve 202. When a fluid is introduced into the fluid channel 205, a portion of the fluid enters microchannel 204. Flow sensor 201 measures the velocity of the fluid and outputs the velocity to controller 203. Controller 203 controls the velocity by adjusting microvalve 202 according to the velocity so measured.
In a mass flow controller as described above, requirements include accuracy of control to be 0.05% the full scale of the flow and response time to be under 0.5 sec. If full scale of the flow is 200 sccm, accuracy should be 0.1 sccm. While the "response time" shall include operation time consumed in the controller 203, the response time of the flow sensor should be under 0.25 sec.
FIG. 3 illustrates the relation between measured velocity and resolution in a thermal pulsed micro flow sensor with 600:1 bypass ratio. As shown in this figure, when the measurable scale of a flow sensor is expanded, its resolution will be decreased. If requirement in accuracy is 0.1 sccm, measurable scale of the flow sensor will be limited to under 23.3 sccm. It is possible to expand the measurable scale by expanding the sectional area of the microchannel. This, however, will lengthen the reaction time because of lower resolution, especially when flow speed is relatively low.
In order to solve the conflict, disclosed was an improved flow sensor in U.S. Pat. No. 5,533,412 (Jerman et al.) wherein the bypass channel has several sections, each section having different cross sectional area. Several thermal sensors are positioned in each section respectively. When the flow speed is higher, thermal sensors in a section with larger cross sectional area are used. And vice versa.
The flow sensor disclosed by Jerman et al. may be applied to a variety of velocities. It, however, has several drawbacks. First, due to the design of the multiple sections, the space required for a flow sensor will be expanded. The flow sensor so prepared will then be bulky and its manufacture cost is increased. As the cross sectional area of every section is different from that of others, sizes of heaters and thermal sensors shall be adjusted from section to section. This will bring difficulties in the manufacture process. Last but not least, since size of heaters and thermal sensors varies from section to section, special circuit is required to solve the difference in resistance in each section. Design of the flow sensor will thus become complex.
It is thus a need in the industry to have a simplified micro flow sensor that is applicable to a relatively larger scale of measurement. It is also a need to have a micro flow sensor with higher resolutions.